This invention relates to disposable, thin-wall, plastic lids and more particularly comprises a new and improved lid having a stacking facility which cooperates with other identical or very similar lids to form a very stable lid stack and to prevent jamming when axial loads are applied to them.
In the past 15 years considerable research has been directed to the design of stacking facilities in thin-wall, nestable articles such as lids and containers, to prohibit them from jamming when a plurality of such articles of identical form are arranged in a stack and subjected to axially applied loads. This research has been prompted by the rapidly growing use of disposable thermoformed plastic lids and containers in fast food establishments, vending machines and automatic filling equipment.
Many of the stacking facilities for thin wall, thermoformed, plastic articles are in the form of undercuts wherein the upper margin or shoulder of the undercut is supported at its inner edge by an upwardly and inwardly inclined intermediate wall. The undercut creates a positive interference between the upper and lower shoulders of adjacent containers, which interference exceeds the thickness of the wall of the article. There are certain disadvantages, however, to undercut stacking rings of that type. For example, the undercuts make it difficult to remove the articles from the mold cavities after they are formed, and special stripping mechanisms are frequently necessary to eject the articles from the cavities. Difficulties are also encountered in actually forming the details of the undercuts, and if the details are not formed well, the articles may jam together when an axial load is applied. In lids, it is the general practice to provide such undercuts in the skirt and because the skirts of the lids are particularly thin and lack rigidity, the stacking undercuts do not always perform well.
The stacking facilities of the general character found in the prior art, and particularly in flush-type lids designed for cold drink cups, do not create a very stable stack. Even though the stacking facilities may prevent axial jamming, they do not dependably maintain the lids in vertical alignment; rather, they permit the lids to slip sideways to create a rather sloppy pile of lids when the lids are not confined in a magazine or some special storage container. This is particularly troublesome when the lids are designed for over-the-counter use in fast food establishments where they are normally placed on a counter without any container about them so as to be readily accessible to those working at the counter.
One important object of this invention is to provide a stacking facility particularly designed for thin wall lids, which facility is free of undercuts and which nevertheless maximizes the interference of adjacent lids in a stack to prevent jamming.
Another important object of this invention is to provide a stacking facility particularly useful in lids, which lends great lateral stability to a stack of lids so that the lids may be vertically stacked by themselves in substantial numbers without falling over. Such a stacking facility is particularly useful in cold drink lids that have flat or only slightly recessed closure walls as opposed to those lids which have deeply recessed walls as are commonly used on hot drink cups.
Another important object of this invention is to provide a thin-wall disposable plastic lid specially fluted about the peripheral portion thereof so as to lend stiffness to what normally is the thinnest part of the lid made by the thermoforming technique.
Another important object of this invention is to provide a thin wall, stackable, disposable, plastic, thermoformed lid having a fluted configuration at the outer skirt which provides axial stiffness and circumferential flexibility.
Another important object of this invention is to provide a lid which will engage the bottom of a cup stacked on top of it to prevent the cup from slipping sideways off the lid so that filled and capped cups may be stacked on top of one another.
To accomplish these and other objects, the thin wall disposable plastic lid of this invention includes a stacking ring in the form of a circular recess formed on the closure wall, which recess has formed in its bottom wall a plurality of circumferentially spaced downwardly extending feet. When two identical lids are stacked together, the feet of the upper lid rest on the bottom wall of the recess of the lower lid to prevent the lids from jamming and/or moving sideways with respect to one another.